Many shoulder disorders are related to the mechanics of the glenohumeral joint, and much of shoulder mechanics is dictated by the shallow geometry of the glenoid fossa. A mathematical analysis of the glenohumeral joint indicates that angulation of the glenoid in the plane of the scapula, which we term "glenoid inclination," affects the ability of the rotator cuff muscles to prevent superior humeral head translation. Superior translation of the humeral head is undesirable because it can produce compression force on the supraspinatus tendon of the rotator cuff. We hypothesize a structure-function relationship between glenoid angle and superior translation of the humeral head. This research will use computational models and in vivo experiments to address the global hypothesis that glenoid inclination angle affects the frequency of superior translation events. There are two Specific Aims of the project: (1) to test the hypothesis that the length-tension and force-velocity properties of muscle affects EMG-based estimates of shoulder muscle force during lifting and reaching activities; and (2) to test the hypothesis that glenoid inclination angle affects the probability of superior humeral head migration during simulated lifting and reaching. In the first Specific Aim, in vivo experiments will be performed to estimate muscle contraction forces from EMG signals during arm motions. The first Specific Aim will enhance to model to be used in the second Specific Aim. In the second Specific Aim, a stochastic computational model will be used to investigate the relationship between glenoid inclination and the probability of superior humeral head migration. The project is significant because it will provide a scientific basis for therapeutic interventions. The project has three innovative aspects: (1) it challenges an existing paradigm of clinical orthopaedic thought about glenohumeral joint function, (2) it introduces a modeling method for investigating the stochastic behavior of the glenohumeral joint due to variability in muscle contraction force, and (3) it will provide improved estimates of glenohumeral loading during functional three-dimensional activities. The project will be a multi-institutional collaboration, although the majority of the work will be performed at the Orthopaedic Research Laboratories of the University of Michigan. [unreadable] [unreadable]